Flat batteries such as button type batteries and coin type batteries have widely been used as power sources for small or thin devices such as wrist watches and hearing aids or IC cards.
FIG. 16 illustrates a conventional coin type battery, which is a typical flat battery. A positive electrode pellet 32 and a negative electrode pellet 33 of disk-like form are placed opposite each other with a separator 34 interposed between them inside a negative electrode case 35 of circular half shell form having a stepped lateral wall. Liquid electrolyte is injected into this case, a positive electrode case 31 is coupled to the open end of the negative electrode case 35 with a gasket 36 fitted therebetween, and the open end edge of the positive electrode case 31 is crimped or bent inwards so as to tightly seal the inner space formed between the positive and negative electrode cases 31 and 35.
Coin type batteries having such a structure wherein one positive electrode pellet 32 and one negative electrode pellet 33 are arranged opposite each other can have only a small continuous discharge current of several tens of mA because of the small reaction area where the positive and negative electrode plates face to each other, so that they have only been used for devices that require a small load current.
To achieve a large discharge current, it is necessary to increase the facing area between positive and negative electrode plates. Electrode plate groups having a structure that allows the reaction area to be increased such as a layered structure or a coiled structure have widely been used for batteries other than coin type batteries. Japanese Patent Laid-Open Publication No. 2000-164259 discloses a flat battery incorporating one such electrode plate group of a layered or coiled structure accommodated in a flat battery case having a small height so as to achieve higher discharge current. This flat battery accommodates the electrode plate group, which is formed into a thin prismatic shape by the coiled or layered structure, in a prismatic positive electrode case.
This electrode plate group of coiled or layered structure can not be adopted to coin type batteries that have a circular plan, because accommodating rectangular electrode plate group in a circular positive electrode case results in insufficient battery capacity because of the low volume efficiency of the battery.
Another problem in constructing a battery using non-aqueous electrolyte such as a lithium ion rechargeable battery in a flat shape is that, if the electrode plate group of the coiled structure contains water, it generates hydrogen gas during the initial charge and discharge. Water may also cause swelling of the electrode plates, which will lead to swelling of the outer case.
The gas generated during the initial period of use may change the thickness of the coiled electrode plate group and cause variations in the pressure distribution in the group. This may lead to buildup of ions in low-pressure portions or cause swelling of the outer case.
The positive and negative electrode plates forming the electrode plate group are electrically connected to respective cases by pressure contact or by welding. For the electrode plate group of a coiled structure, the connection can be achieved with highest reliability by welding. Spot welding, for example, is carried out as shown in FIG. 17: A negative electrode lead 45 is pressed against a negative electrode case 44 by a pair of welding electrodes 40, 41, and welding current is applied thereacross from a welding power source 48. However, sparks thrown and dust scattered during the welding may cause internal short-circuiting or buildup of ions.
An object of the present invention is to provide a flat battery that resolves the above problems found in flat batteries accommodating electrode plate groups of a coiled structure within thin spaces so as to increase discharge capacity, and a manufacturing method of this flat battery.